The long-haul communication capacity of HF links relies on the reflection of the HF waves varying within the range [2, 30 MHz], on the ionospheric layers. The ionospheric layers are not stable in time and space, which leads to strong variations of the propagation channel. To this channel instability are also added any scrambling factors, intentional or not, in particular at night when the passing HF spectrum is less great.
Although it is unstable, this channel does however offer the benefit of allowing long-haul communications without the need to previously deploy a complicated or costly infrastructure, unlike satellite communications for example. Although it offers these advantageous properties, the HF channel is currently used only for fairly simple services (telegraphy, speech, low-bit rate file transfers) because of the low bit rates offered. Efforts are therefore currently being made to increase the bit rates offered by the HF links, to obtain bit rates that are sufficient to allow the transmission of different data types (speech, file transfer, messaging, videotelephony, imaging on demand, for example).
One problem that arises, in the context of a multiservice use for HF band transmission, is linked to the half duplex operation, the latter generally being coupled with fairly lengthy interleaving times to fight against the very significant instantaneous fading effects to which the HF propagation channel is subject.
One known solution is to choose an interleaver of small size and a fast half-duplex switchover, corresponding to raw performance levels that are less good but with a greater responsiveness that is necessary for applications with strong real-time constraint such as speech.
Another solution consists in using an interleaver of large size and a slower half-duplex switchover, for example of 9 seconds frame duration in very long interleaving in the stanag 4539 standard that is known to those skilled in the art, when very good performance levels in terms of error rate are expected, and the latency or jitter constraints are low, as is the case for data transmission, for example.
In the case of a multiservice approach, it is known practice from the prior art to process the services sequentially, and thus to best adapt the communication conditions to each of them. This approach does not make it possible to multiplex the services. The real-time constraints of a given service will not therefore be able to be served without stopping the current service if it is activated when a communication for another service is already established. This, furthermore, prevents particular uses, such as, for example, switching over to a speech channel that might be set to standby in parallel with a data transmission without interrupting the latter. This also dictates a significant delay before the transmission of the acknowledgements or signaling information between the stations handling the communication. In the case involving serving the streams sequentially, it is not possible to conduct a plurality of communications in parallel on one and the same link and therefore it is necessary to await the end of the communication for the next service to be established.
It is also known practice to process the services jointly, by taking into account the strongest constraints in each of the areas (latency, jitter, error rate) if possible or, failing this, the strongest constraints concerning the real-time behavior (latency, jitter) and use the ARQ retransmission to successfully pass on the most sensitive service or services. The strong constraint imposed on the duration of the half-duplex switchover will greatly degrade the performance levels in terms of error rate, rendering the multiplexing ineffective.
By way of example, it may be necessary to divide the served bit rate by two or more by changing from a data transmission type mode with very long interleaver with the standard MIL STD 188-110B (target TEB=10−5) to a speech-type mode with short interleaver with the standard MIL STD 188-110A (target TEB=10−3).
FIG. 1 reviews the data processing principles implemented in a radio communication system. The data D1 arrive at the receiver 1 in a queue 10 to be processed by a scheduler 11, possibly with the retransmissions 10′ coming from the retransmission mechanism (ARQ) securing the link. The scheduler processes these data and prepares the frame 12 for sending to the physical layer comprising the correcting coding 13, interleaving and modulation 14. The duly modulated frame is then transmitted over the HF channel before being received by the receiver 2 comprising demodulation and de-interleaving 15 and correcting decoding 16 before reaching the counterpart of the scheduler 17 which reconstructs the packets, to obtain the data D2.
FIG. 2 schematically represents the sequencing of the stream processing steps using a prior art method. FIG. 2 schematically shows the link layer and the physical layer.
At the link layer level, the data stream 20 to be transmitted is first of all segmented 21 into a plurality of blocks Bi (unitary cell for the process of securing the ARQ link, which is an information retransmission mechanism, or “Automatic Repeat reQuest”). Then, the method prepares the frame 22 by adding, for example, headers, by setting the frame to the format required for the transmission, then adds 23 an error correcting coding. The data of the frame will then be interleaved 24 according to a method known to those skilled in the art. The method will then transmit the frame 25 according to a sequence of transmission of one or more frames (251, 252, etc.) followed by a TX/RX (transmit/receive) half-duplex switchover, 253, then a sequence of reception of one or more frames (254, etc.). In this exemplary embodiment, the data are interleaved after the ARQ step, the frame formatting step and the step of introduction of a correcting coding. The bit rate is adapted through the choice of modulation and coding. The interleaving is adapted through the duration of the interleaver to the needs of the service concerned. Typically, speech will accept a target error rate of the abovementioned 10−3 class, but will require the use of a short interleaver (less 1 s) whereas a data transfer targets an error rate below 10−5 but accepts the use of long interleavers (typically greater than 9 s).
One of the objectives of the present patent application is to offer a system and a method that make it possible to effectively multiplex different services having different service qualities on one and the same communication link, and do so without degrading the transmission performance levels in terms of overall useful bit rate transmitted.
The following definitions will be used hereinbelow in the description.
A stream is a set of data originating from application layers and which are brought to the transmission system to be sent to one or more receivers. The data of this stream, arriving for example in packet form (as in the case of applications over IP), are stored for processing in queues. The different mechanisms of the transmission system come to take the data from the queues according to their capacity to prepare the frames which will be transmitted. In practice, there are different levels of queues between the different processing instances of the radio system, even though, for simplicity, queues are more often mentioned with reference to those that exist at the level of the ARQ process, which introduces specific queues for the retransmissions. The frames are made up of a set of data with header, which are addressed from a sender to one or more remote receivers. A frame conveys a portion of the information of a data stream.